Implantable medical devices, such as pacemakers, defibrillators, and cardioverters (collectively referred to herein as implantable cardiac stimulating devices), are designed to monitor and stimulate the heart of a patient that suffers from a cardiac arrhythmia. Using leads connected to a patient's heart, these devices typically stimulate the cardiac muscles by delivering electrical pulses in response to measured cardiac events that are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart's regular rhythm.
Implantable cardiac stimulating devices can treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy, shape, location, and frequency of the therapeutic pulses. The adjustable parameters are usually defined in a computer program stored in a memory of the implantable device. The program (which is responsible for the operation of he implantable device) can be defined or altered telemetrically by a medical practitioner using an external implantable device programmer.
Modern programmable pacemakers, the most commonly used implantable devices, are generally of two types: (1) single-chamber pacemakers, and (2) dual-chamber pacemakers. In a single-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, a single-chamber of the heart (e.g., either the right ventricle or the right atrium). In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, two chambers of the heart (e.g. both the right atrium and the right ventricle). The left atrium and left ventricle can also be paced, provided that suitable electrical contacts are effected therewith.
In general, both single and dual-chamber pacemakers are classified by type according to a three letter code. In this code, the first letter identifies the chamber of the heart that is paced (i.e., the chamber where a stimulation pulse is delivered)--with a "V" indicating the ventricle, an "A" indicating the atrium, and a "D" (dual) indicating both the atrium and ventricle. The second letter of the code identifies the chamber where cardiac activity is sensed, using the same letters to identify the atrium or ventricle or both, and where an "O" indicates that no sensing takes place.
The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting ("I") response, where a stimulation pulse is delivered to the designated chamber after a set period of time unless cardiac activity is sensed during that time, in which case the stimulation pulse is inhibited; (2) a Trigger ("T") response, where a stimulation pulse is delivered to the designated chamber of the heart a prescribed period after a sensed event; or (3) a Dual ("D") response, where both the Inhibiting mode and Trigger mode are evoked, inhibiting in one chamber of the heart and triggering in the other.
A fourth letter, "R", is sometimes added to the code to signify that the particular mode identified by the three letter code is rate-responsive, where the pacing rate may be adjusted automatically by the pacemaker based on one or more physiological factors such as blood oxygen level or the patient's activity level.
Modern pacemakers also have a great number of adjustable parameters that must be tailored to a particular patient's therapeutic needs. One adjustable parameter of particular importance in pacemakers is the pacemaker's stimulation energy. "Capture" is defined as a cardiac response to a pacemaker stimulation pulse. When a pacemaker stimulation pulse stimulates either a heart atrium or a heart ventricle during an appropriate portion of a cardiac cycle, it is desirable to have the heart properly respond to the stimulus provided. Every patient has a "capture threshold" which is generally defined as the minimum amount of stimulation energy necessary to effect capture. Capture should be achieved at the lowest possible energy setting yet provide enough of a safety margin so that, should a patient's threshold increase, the output of an implantable pacemaker, i.e., the stimulation energy, will still be sufficient to maintain capture. Dual-chamber pacemakers may have differing atrial and ventricular stimulation energy that correspond to atrial and ventricular capture thresholds, respectively.
The earliest pacemakers had a predetermined and unchangeable stimulation energy, which proved to be problematic because the capture threshold is not a static value and may be affected by a variety of physiological and other factors. For example, certain cardiac medications may temporarily raise or lower the threshold from its normal value. In another example, fibrous tissue that forms around pacemaker lead heads within several months after implantation may raise the capture threshold.
As a result, some patients eventually suffered from loss of capture as their pacemakers were unable to adjust the pre-set stimulation energy to match the changed capture thresholds. One attempted solution was to set the level of stimulation pulses fairly high so as to avoid loss of capture due to a change in the capture threshold. However, this approach resulted in some discomfort to patients who were forced to endure unnecessarily high levels of cardiac stimulation. Furthermore, such stimulation pulses consumed extra battery resources, thus shortening the useful life of the pacemaker.
When programmable pacemakers were developed, the stimulation energy was implemented as an adjustable parameter that could be set or changed by a medical practitioner. Typically, such adjustments were effected by the medical practitioner using an external programmer capable of communication with an implanted pacemaker via a magnet applied to a patient's chest or via telemetry. The particular setting for the pacemaker's stimulation energy was usually derived from the results of extensive physiological tests performed by the medical practitioner to determine the patient's capture threshold, from the patient's medical history, and from a listing of the patient's medications. While the adjustable pacing energy feature proved to be superior to the previously known fixed energy, some significant problems remained unsolved. In particular, when a patient's capture threshold changed, the patient was forced to visit the medical practitioner to adjust the pacing energy accordingly.
To address this pressing problem, pacemaker manufacturers have developed advanced pacemakers that are capable of determining a patient's capture threshold and automatically adjusting the stimulation pulses to a level just above that which is needed to maintain capture. This approach, called "autocapture", improves the patient's comfort, reduces the necessity of unscheduled visits to the medical practitioner, and greatly increases the pacemaker's battery life by conserving the energy used to generate stimulation pulses.
However, many of these advanced pacemakers require additional circuitry and/or special sensors that must be dedicated to capture verification. This requirement increases the complexity of the pacemaker system and reduces the precious space available within a pacemaker's casing, and also increases the pacemaker's cost. As a result, pacemaker manufacturers have attempted to develop automatic capture verification techniques that may be implemented in a typical programmable pacemaker without requiring additional circuitry or special dedicated sensors.
A common technique used to determine whether capture has been effected is monitoring the patient's cardiac activity and searching for the presence of an "evoked response" following a stimulation pulse. The evoked response is the response of the heart to application of a stimulation pulse. The patient's heart activity is typically monitored by the pacemaker by keeping track of the stimulation pulses delivered to the heart and examining, through the leads connected to the heart, electrical signals that are manifest concurrent with depolarization or contraction of muscle tissue (myocardial tissue) of the heart. The contraction of atrial muscle tissue is evidenced by generation of a P-wave, while the contraction of ventricular muscle tissue is evidenced by generation of an R-wave (sometimes referred to as the "QRS" complex).
When capture occurs, the evoked response is an intracardiac P-wave or R-wave that indicates contraction of the respective cardiac tissue in response to the applied stimulation pulse. For example, using such an evoked response technique, if a stimulation pulse is applied to the atrium (hereinafter referred to as an A-pulse), any response sensed by atrial sensing circuits of the pacemaker immediately following application of the A-pulse is presumed to be an evoked response that evidences capture of the atria.
However, it is for several reasons very difficult to detect a true evoked response. First, because the atrial evoked response is a relatively small signal, it may be obscured by a high energy A-pulse and therefore difficult to detect and identify. Second, the signal sensed by the pacemaker's sensing circuitry immediately following the application of a stimulation pulse may be not an evoked response but noise--either electrical noise caused, for example, by electromagnetic interference, or myocardial noise caused by random myocardial or other muscle contraction.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult for which to compensate because it is usually present in varying degrees, is lead polarization. A lead/tissue interface is that point at which an electrode of the pacemaker lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead/tissue interface due to application of an electrical stimulation pulse, such as an A-pulse, across the interface. Unfortunately, because the evoked response is sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to herein as an "afterpotential", formed at the electrode can corrupt the evoked response that is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time.
In each of the above cases, the result may be a false positive detection of an evoked response. Such an error leads to a false capture indication, which in turn leads to missed heartbeats--a highly undesirable and potentially life-threatening situation. Another problem results from a failure by the pacemaker to detect an evoked response that has actually occurred. In that case, a loss of capture is indicated when capture is in fact present--also an undesirable situation that will cause the pacemaker to unnecessarily invoke the pacing energy determination function in a chamber of the heart.
Automatic pacing energy determination is only invoked by the pacemaker when loss of atrial or ventricular capture is detected. An exemplary prior art automatic atrial pacing energy determination procedure is performed as follows. When loss of atrial capture is detected, the pacemaker increases the A-pulse output level to a relatively high predetermined testing level at which capture is certain to occur, and thereafter decrements the output level until atrial capture is lost. The atrial pacing energy is then set to a level slightly above the lowest output level at which atrial capture was attained. Thus, atrial capture verification is of utmost importance in proper determination of the atrial pacing energy.
When an atrial stimulation pulse is properly captured in the atrium, a subsequent ventricular contraction results in an R-wave which may be sensed through an atrial lead, in patients with intact atrioventricular ("AV") conduction, as a "far-field" signal. The far-field R-wave confirms successful atrial capture because the ventricular contraction only occurs after a properly captured atrial stimulation pulse. Previously known pacemakers have ignored this useful phenomenon because previously known single-chamber atrial pacemakers and dual-chamber pacemakers programmed to operate in an atrial mode purposefully do not sense ventricular activity through the atrial lead for a particular period of time (i.e., the "refractory" period) after delivery of the atrial stimulation pulse. Furthermore, the polarization signal formed at the atrial lead electrode may obscure and/or distort the far-field R-wave signal, even if it were sensed.
It would thus be desirable to provide a system and method for enabling the pacemaker to automatically and accurately perform atrial capture verification by sensing and identifying a far-field R-wave that occurs only after delivery by the pacemaker of a successfully captured atrial stimulation pulse. It would also be desirable to provide a system and method for reducing the negative effect of polarization and noise on capture verification by automatically isolating such negative effects from the identified far-field R-wave signal. It would further be desirable to enable the pacemaker to perform atrial capture verification without requiring dedicated circuitry and/or special sensors.